This section provides background information related to the present disclosure which is not necessarily prior art.
Focused ultrasound in high-intensity has attracted great attention because it involves a variety of interesting phenomena such as shock waves, cavitation bubbles, and local heat deposition. These mechanisms have been broadly employed in modern acoustics for fundamental understanding of nonlinear acoustic effects, thermal therapies, shock wave lithotripsy, and intra-membrane drug delivery. High-intensity focused ultrasound (HIFU) has been generated by using common piezoelectric transducers, which are usually operated around low frequency (around 1 to 2 MHz). This low frequency limits spatial resolution of an applied focal spot to a range of several mm to a few tens of mm (in an axial direction). This is insufficient for high resolution applications requiring sub-millimeter accuracy, e.g., for physical therapies. Furthermore, possible damage should be minimized over a surrounding volume to the focal spot, particularly in surgical applications. These considerations make it desirable to have a high-frequency ultrasound that can be tightly focused, which is currently not available for conventional HIFU systems.
Optoacoustic generation is one of the most effective ways to obtain high-frequency ultrasound. In one-dimensional structures, a frequency spectrum of the generated ultrasound can closely replicate that of an original laser pulse used for excitation. Nanosecond laser pulses are commonly available, which are sufficient to generate ultrasonic pulses with several tens of MHz of frequency spectra. Such a frequency range is typically sufficient for achieving micro-scale resolution ultrasonic imaging and non-destructive evaluations. However, practical utilization of such light-generated high-frequency ultrasound has been limited to proximity imaging because of weak pressure output and frequency-dependent attenuation during propagation, which increases with the acoustic frequency and propagation distance. For long-range imaging over several centimeters and for therapeutic applications of the light-generated ultrasound such as lithotripsy and surgical techniques like ablation aiming at higher resolution, high-efficiency optoacoustic materials are required to achieve high-amplitude and high-intensity ultrasound over the high-frequency range.
As optoacoustic emission sources, thin metallic coatings on solid substrates have been used as common reference materials. Such metal thin films (typically about 100 nm in thickness) are suitable for high-frequency ultrasound sources, because an acoustic transit time over the thin films can be much shorter than the temporal width of laser pulses. However, optoacoustic conversion efficiency in the metal is poor, mainly because of low light absorption and low thermal expansion. In addition, acoustic impedances of the metals do not match with those of surrounding liquids (e.g. water), which results in inefficient pressure transfer. For highly efficient transmitters of strong and high-frequency ultrasound generation, it would be desirable to have a transmitter capable of high optical absorption, high thermal expansion, fast thermal transition, acoustic impedance matching with a surrounding medium, and a geometrically thin structure for less acoustic attenuation within the source, together with less broadening in a temporal pulse shape, by way of non-limiting example.